INITIATION OF PROTO-FRANCISCAN SUBDUCTION ALONG A TRANSFORM FAULT ZONE: EVIDENCE FROM SERPENTINITE MELANGE OF THE COAST RANGE OPHIOLITE COMPLEX

The initiation of subduction is a mysterious process that cannot be observed directly, but must be inferred from the rock record after subduction is well established. There are many approaches possible for subduction zones that are still active, but paleo-subduction zones present special challenges, since their geodynamic setting can no longer be directly observed. In this study we examine petrology of the Tehama-Colusa serpentinite mélange, which underlies the Coast Range ophiolite in northern California and separates it from high P/T metamorphic rocks of the Franciscan complex, to understand better how these assemblages formed and the origin of Franciscan subduction.

The Tehama-Colusa serpentinite mélange comprises blocks of basalt, chert, sedimentary rocks, and peridotite (harzburgite and lherzolite) in a sheared serpentinite matrix. It has been interpreted both as an accreted fracture zone terrane and as a subduction-derived mélange belt. Our data show that the mélange matrix represents hydrated refractory peridotites with fore-arc affinities, and that blocks within the mélange consist largely of upper plate lithologies (refractory fore-arc harzburgite, arc volcanics, arc-derived sediments, and chert with Coast Range ophiolite biostratigraphy). Lower plate blocks within the mélange include oceanic basalts and chert with, rare blueschist and amphibolite. Abyssal lherzolites have trace element signatures that indicate subduction enrichment with fluid mobile elements.

Choi et al (2008) have shown that abyssal peridotite blocks within the mélange have low pyroxene equilibration temperatures that are consistent with formation in a fracture zone setting. However, the current mélange reflects largely upper plate lithologies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large offset transform fault/fracture zone that evolved into subduction zone mélange complex. The nucleation of subduction zones along former transform boundaries has long been proposed for both modern arc systems (e.g., Casey & Dewey 1984; Bloomer et al 1995) and for the Franciscan-CRO system (Stern & Bloomer 1992). Our data support this interpretation and document more fully how this mechanism is expressed by mixing within the evolving serpentinite melange.